The active optical fibers are prepared by drawing glass into optical fiber from a fiber preform, which fiber preform can be created in several different ways. A generally used manner for preparing a fiber preform is to grow glass material around a mandrel, or a corresponding substrate arranged to rotate, by flame hydrolysis deposition, FHD. When the above-mentioned growing is performed from the outer periphery of the fiber preform, it is often in this connection referred to as a so-called OVD method (outer vapour deposition). The FHD method is also applied in forming glass layers required in optical planar waveguides on a planar substrate.
In the FHD method, a hydrogen-oxygen flame is typically used as a thermal reactor, and the glass forming base materials used in the preparation of glass material, for example silicon or germanium tetrachloride, are directed to the burner and the flame typically in a vapour form. The dopants of glass material, such as, for example, erbium, are directed to the burner and the flame typically with carrier gas as vapour or aerosol droplets, which are formed of the liquid containing dopants correspondingly either by vaporizing or by spraying.
Alternatively, according to the solution developed by the applicant, the dopants can be directed all the way to the burner in liquid form and be atomised as aerosol droplets, for example by using hydrogen flow, not until in the immediate vicinity of the flame. This method, which is described more in detail, for example, in the applicant's earlier publication WO 00/20346 and which can be considered a further development of the conventional FHD method, is later referred to as liquid flame spraying.
In the flame functioning as a thermal reactor in the FHD or liquid flame spraying method, the base materials and dopants further form aerosol particles, which aerosol particles are guided onto the substrate to be coated, thus forming a doped porous glass material coating. These aerosol particles are often in literature referred to as “glass soot” in English. When a suitable coating layer of porous glass material has been grown on the mandrel or other substrate, the above-mentioned coating layer is sintered into a dense glass by heat-treating the substrate at an appropriate high temperature.
A so-called solution doping method is also known, in which method a fiber preform grown of only base materials is dipped into a solution containing dopants only after growing the fiber preform, before sintering.
Rare earth metals dissolve poorly into quartz glass and require that, for example, the structure of SiO2-based glass is changed by adding an appropriate oxide to the glass. Oxides suitable for the purpose are, for example, Al2O3, La2O3, Yb2O3, GeO2 or P2O5. Preferably this oxide is aluminium oxide Al2O3, which at the same time increases the refractive index of the glass.
When doping the core of optical fiber (or other waveguide) with a rare earth metal, an increase in the refractive index of the core in relation to the cladding layer is achieved at the same time by means of the aluminium oxide, which is necessary in order for the operating principle of the optical fiber to materialize. In the liquid flame spraying method of the applicant, the aluminium is added by atomising aluminium chloride dissolved in a suitable liquid to the flame. Liquids suitable for the purpose are, for example, water, organic solvents, such as ethanol, methanol, acetone, or mixtures of the above. Correspondingly, nitrate or chloride based sources dissolved in a liquid are used for rare earth metals, such as erbium.
In the growing that takes place by means of the methods described above, when silicate/alumina glass are doped with rare earth metals, one problem is the inhomogeneous distribution of dopants into aerosol particles forming glass coating. This is caused by e.g. the tendency of dopants to form pairs. In a chemical balance, erbium does not dissolve in said materials as individual ions separate from each other. In a gas phase erbium aims to oxidize into form Er2O3 and in a solid phase erbium aims typically to a phase system Al5Er3O12+Al2O3 with aluminium. In other words, with aluminium erbium aims to occur clustered in its own phases. Even though the situation in a glass-like silica/alumina system is more complex than described above, the above discussion offers a good impression on how erbium acts.
Especially when using the liquid flame method, most of aluminium and a majority of erbium aims to remain in the solid residual particle, which is created from a liquid aerosol droplet when it “dries” in the flame, and wherein the abovementioned oxidation of materials into glass forming oxides takes place. Because of this, the fiber preform forming in the process typically includes at least two types of glass soot particles. Firstly, small Si-containing (or Ge-containing) particles, which are formed via condensation from vaporous base materials and the evaporation/drying following it. Secondly, aluminium and erbium containing residual particles, which are typically larger than these Si-particles. Because of these different types of particles, there is a crystallizing tendency in the glass material when it is sintered.
During sintering, a part of the crystals may also melt, which improves the homogeneity of the glass material. However, there is the risk that remaining dopants, especially in the larger residual particles, do not even then dissolve completely in the glass, in which case, when examined on the small scale, the consequence is that the dopants are locally inhomogeneously parted in the glass material. This weakens the light amplifying properties of the glass.
On the other hand, in the case of a, for example, silicon wafer based planar waveguide, the temperatures used in sintering are more limited than in the case of a fiber preform meant for optical fiber. Thus, unwanted crystals causing scattering unavoidably remain in the ready glass coating even after sintering and because of the inhomogeneous composition of the glass material also the light amplifying properties of glass are unideal.
In all such processes, wherein glass soot particles and especially particles containing dopants are not created substantially directly by condensation via a gas phase, but larger liquid aerosol droplets are an intermediate phase, a problem is that different impurities also remain (encapsulate) in the residual particles forming from aerosol droplets.